OAuth to Account takeover

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Basic Information

OAuth offers various versions, with foundational insights accessible at OAuth 2.0 documentation. This discussion primarily centers on the widely used OAuth 2.0 authorization code grant type, providing an authorization framework that enables an application to access or perform actions on a user's account in another application (the authorization server).

Consider a hypothetical website https://example.com, designed to showcase all your social media posts, including private ones. To achieve this, OAuth 2.0 is employed. https://example.com will request your permission to access your social media posts. Consequently, a consent screen will appear on https://socialmedia.com, outlining the permissions being requested and the developer making the request. Upon your authorization, https://example.com gains the ability to access your posts on your behalf.

It's essential to grasp the following components within the OAuth 2.0 framework:

  • resource owner: You, as the user/entity, authorize access to your resource, like your social media account posts.

  • resource server: The server managing authenticated requests after the application has secured an access token on behalf of the resource owner, e.g., https://socialmedia.com.

  • client application: The application seeking authorization from the resource owner, such as https://example.com.

  • authorization server: The server that issues access tokens to the client application following the successful authentication of the resource owner and securing authorization, e.g., https://socialmedia.com.

  • client_id: A public, unique identifier for the application.

  • client_secret: A confidential key, known solely to the application and the authorization server, used for generating access_tokens.

  • response_type: A value specifying the type of token requested, like code.

  • scope: The level of access the client application is requesting from the resource owner.

  • redirect_uri: The URL to which the user is redirected after authorization. This typically must align with the pre-registered redirect URL.

  • state: A parameter to maintain data across the user's redirection to and from the authorization server. Its uniqueness is critical for serving as a CSRF protection mechanism.

  • grant_type: A parameter indicating the grant type and the type of token to be returned.

  • code: The authorization code from the authorization server, used in tandem with client_id and client_secret by the client application to acquire an access_token.

  • access_token: The token that the client application uses for API requests on behalf of the resource owner.

  • refresh_token: Enables the application to obtain a new access_token without re-prompting the user.


The actual OAuth flow proceeds as follows:

  1. You navigate to https://example.com and select the “Integrate with Social Media” button.

  2. The site then sends a request to https://socialmedia.com asking for your authorization to let https://example.com’s application access your posts. The request is structured as:

  1. You are then presented with a consent page.

  2. Following your approval, Social Media sends a response to the redirect_uri with the code and state parameters:

  1. https://example.com utilizes this code, together with its client_id and client_secret, to make a server-side request to obtain an access_token on your behalf, enabling access to the permissions you consented to:

POST /oauth/access_token
Host: socialmedia.com
...{"client_id": "example_clientId", "client_secret": "example_clientSecret", "code": "uniqueCode123", "grant_type": "authorization_code"}
  1. Finally, the process concludes as https://example.com employs your access_token to make an API call to Social Media to access


Open redirect_uri

The redirect_uri is crucial for security in OAuth and OpenID implementations, as it directs where sensitive data, like authorization codes, are sent post-authorization. If misconfigured, it could allow attackers to redirect these requests to malicious servers, enabling account takeover.

Exploitation techniques vary based on the authorization server's validation logic. They can range from strict path matching to accepting any URL within the specified domain or subdirectory. Common exploitation methods include open redirects, path traversal, exploiting weak regexes, and HTML injection for token theft.

Besides redirect_uri, other OAuth and OpenID parameters like client_uri, policy_uri, tos_uri, and initiate_login_uri are also susceptible to redirection attacks. These parameters are optional and their support varies across servers.

For those targeting an OpenID server, the discovery endpoint (**.well-known/openid-configuration**) often lists valuable configuration details like registration_endpoint, request_uri_parameter_supported, and "require_request_uri_registration. These details can aid in identifying the registration endpoint and other configuration specifics of the server.

XSS in redirect implementation

As mentioned in this bug bounty report https://blog.dixitaditya.com/2021/11/19/account-takeover-chain.html it might be possible that the redirect URL is being reflected in the response of the server after the user authenticates, being vulnerable to XSS. Possible payload to test:


CSRF - Improper handling of state parameter

In OAuth implementations, the misuse or omission of the state parameter can significantly increase the risk of Cross-Site Request Forgery (CSRF) attacks. This vulnerability arises when the state parameter is either not used, used as a static value, or not properly validated, allowing attackers to bypass CSRF protections.

Attackers can exploit this by intercepting the authorization process to link their account with a victim's account, leading to potential account takeovers. This is especially critical in applications where OAuth is used for authentication purposes.

Real-world examples of this vulnerability have been documented in various CTF challenges and hacking platforms, highlighting its practical implications. The issue also extends to integrations with third-party services like Slack, Stripe, and PayPal, where attackers can redirect notifications or payments to their accounts.

Proper handling and validation of the state parameter are crucial for safeguarding against CSRF and securing the OAuth flow.

Pre Account Takeover

  1. Without Email Verification on Account Creation: Attackers can preemptively create an account using the victim's email. If the victim later uses a third-party service for login, the application might inadvertently link this third-party account to the attacker's pre-created account, leading to unauthorized access.

  2. Exploiting Lax OAuth Email Verification: Attackers may exploit OAuth services that don't verify emails by registering with their service and then changing the account email to the victim's. This method similarly risks unauthorized account access, akin to the first scenario but through a different attack vector.

Disclosure of Secrets

Identifying and protecting secret OAuth parameters is crucial. While the client_id can be safely disclosed, revealing the client_secret poses significant risks. If the client_secret is compromised, attackers can exploit the identity and trust of the application to steal user access_tokens and private information.

A common vulnerability arises when applications mistakenly handle the exchange of the authorization code for an access_token on the client-side rather than the server-side. This mistake leads to the exposure of the client_secret, enabling attackers to generate access_tokens under the guise of the application. Moreover, through social engineering, attackers could escalate privileges by adding additional scopes to the OAuth authorization, further exploiting the application's trusted status.

Client Secret Bruteforce

You can try to bruteforce the client_secret of a service provider with the identity provider in order to be try to steal accounts. The request to BF may look similar to:

POST /token HTTP/1.1
content-type: application/x-www-form-urlencoded
content-length: 135
Connection: close


Referer Header leaking Code + State

Once the client has the code and state, if it's reflected inside the Referer header when he browses to a different page, then it's vulnerable.

Access Token Stored in Browser History

Go to the browser history and check if the access token is saved in there.

Everlasting Authorization Code

The authorization code should live just for some time to limit the time window where an attacker can steal and use it.

Authorization/Refresh Token not bound to client

If you can get the authorization code and use it with a different client then you can takeover other accounts.

Happy Paths, XSS, Iframes & Post Messages to leak code & state values

Check this post

AWS Cognito

In this bug bounty report: https://security.lauritz-holtmann.de/advisories/flickr-account-takeover/ you can see that the token that AWS Cognito gives back to the user might have enough permissions to overwrite the user data. Therefore, if you can change the user email for a different user email, you might be able to take over others accounts.

# Read info of the user
aws cognito-idp get-user --region us-east-1 --access-token eyJraWQiOiJPVj[...]

# Change email address
aws cognito-idp update-user-attributes --region us-east-1 --access-token eyJraWQ[...] --user-attributes Name=email,Value=imaginary@flickr.com
    "CodeDeliveryDetailsList": [
            "Destination": "i***@f***.com",
            "DeliveryMedium": "EMAIL",
            "AttributeName": "email"

For more detailed info about how to abuse AWS cognito check:

Abusing other Apps tokens

As mentioned in this writeup, OAuth flows that expect to receive the token (and not a code) could be vulnerable if they not check that the token belongs to the app.

This is because an attacker could create an application supporting OAuth and login with Facebook (for example) in his own application. Then, once a victim logins with Facebook in the attackers application, the attacker could get the OAuth token of the user given to his application, and use it to login in the victim OAuth application using the victims user token.

Therefore, if the attacker manages to get the user access his own OAuth application, he will be able to take over the victims account in applications that are expecting a token and aren't checking if the token was granted to their app ID.

Two links & cookie

According to this writeup, it was possible to make a victim open a page with a returnUrl pointing to the attackers host. This info would be stored in a cookie (RU) and in a later step the prompt will ask the user if he wants to give access to that attackers host.

To bypass this prompt, it was possible to open a tab to initiate the Oauth flow that would set this RU cookie using the returnUrl, close the tab before the prompt is shown, and open a new tab without that value. Then, the prompt won't inform about the attackers host, but the cookie would be set to it, so the token will be sent to the attackers host in the redirection.

SSRFs parameters

Check this research For further details of this technique.

Dynamic Client Registration in OAuth serves as a less obvious but critical vector for security vulnerabilities, specifically for Server-Side Request Forgery (SSRF) attacks. This endpoint allows OAuth servers to receive details about client applications, including sensitive URLs that could be exploited.

Key Points:

  • Dynamic Client Registration is often mapped to /register and accepts details like client_name, client_secret, redirect_uris, and URLs for logos or JSON Web Key Sets (JWKs) via POST requests.

  • This feature adheres to specifications laid out in RFC7591 and OpenID Connect Registration 1.0, which include parameters potentially vulnerable to SSRF.

  • The registration process can inadvertently expose servers to SSRF in several ways:

    • logo_uri: A URL for the client application's logo that might be fetched by the server, triggering SSRF or leading to XSS if the URL is mishandled.

    • jwks_uri: A URL to the client's JWK document, which if maliciously crafted, can cause the server to make outbound requests to an attacker-controlled server.

    • sector_identifier_uri: References a JSON array of redirect_uris, which the server might fetch, creating an SSRF opportunity.

    • request_uris: Lists allowed request URIs for the client, which can be exploited if the server fetches these URIs at the start of the authorization process.

Exploitation Strategy:

  • SSRF can be triggered by registering a new client with malicious URLs in parameters like logo_uri, jwks_uri, or sector_identifier_uri.

  • While direct exploitation via request_uris may be mitigated by whitelist controls, supplying a pre-registered, attacker-controlled request_uri can facilitate SSRF during the authorization phase.

OAuth providers Race Conditions

If the platform you are testing is an OAuth provider read this to test for possible Race Conditions.


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